I'd think the age level of you intended audience would come into play here. This may be why there is no single standard. A new reader would find 500 pages daunting even with the limited text on each page a GN has, but the YA audience would be fine with that. I think I'd look at popular recent samples for the age level I was considering to see how this was handled there, but there may be genre differences as well as age level differences, so try to find work similar to your own in some way.